1. Field of the Invention
The present invention relates to a differential amplifying circuit, and, in particular, to a differential amplifying circuit which outputs a signal in accordance with a difference between two input signals.
2. Description of the Related Art
FIG.1 shows a circuit diagram of an example of a differential amplifying circuit in the related art.
The differential amplifying circuit 1 in the related art outputs an output signal in accordance with the difference between a first input voltage and a second input voltage. The differential amplifying circuit 1 includes NPN transistors Q1 and Q2 which input the first and second input voltages, respectively, a constant-current source 2 which draws a current from the transistors Q1 and Q2 and resistors R1, R2, R3 and R4.
The base of the transistor Q1 is connected to a first input terminal TIN1 to which the first input voltage is applied. The collector of the transistor Q1 is connected to a power source 3 through the resistor R1, and the emitter of this transistor is connected to the constant-current source 2 through the resistor R2. The transistor Q1 draws a current from the collector in accordance with the first input voltage, and supplies the current to the constant-current source 2. A first output terminal TOUT1 is connected between the collector of the transistor Q1 and the resistor Ri, and the output signal in accordance with the first input voltage applied to the base of the transistor Q1 is output to the first output terminal TOUTl.
The base of the transistor Q2 is connected to a second input terminal TIN2 to which the second input voltage is applied. The second input voltage is applied to the transistor Q2 through the second input terminal TIN2. The collector of the transistor Q2 is connected to the power source 3 through the resistor R3, and the emitter of this transistor is connected to the constant-current source 2 through the resistor R4. The transistor Q2 draws a current from the collector in accordance with the second input voltage, and supplies the current to the constant-current source 2 to which the transistor Q1 also supplies the current as mentioned above. A second output terminal TOUT2 is connected between the collector of the transistor Q2 and the resistor R3. The transistor Qi supplies the current in accordance with the first input voltage to the constant-current source 2, and the current drawn from the emitter of the transistor Q2 to the constant-current source 2 is controlled. Thereby, the output voltages in accordance with the difference between the first input voltage and the second input voltage are output.
FIG.2 shows a circuit diagram of one example of a differential amplifying circuit of a low-voltage driving type in the related art. The same reference numerals are given to the parts that are the same as those shown in FIG.1 and the descriptions thereof will be omitted.
Instead of the constant-current source 2, and resistors R2 and R4 of the circuit of FIG.1, in the differential amplifying circuit 11, a constant-current source 4 is connected to the emitter of an NPN transistor Q1, a constant-current source 5 is connected to the emitter of an NPN transistor Q2. A resistor R11 is connected between the connection point of the emitter of the transistor Q1 and the constant-current source 4, and the connection point of the emitter of the transistor Q2 and the constant-current source 5. In the resistor R11, the differential voltage between the first and second input voltages is generated.
In the circuit shown in FIG.2, because the emitter of each of the transistors Q1 and Q2 has no series resistor connected thereto, it is possible to reduce the voltage necessary to drive the circuit.
FIG.3 shows a circuit diagram of one example of a differential amplifying circuit having linear input and output characteristics in the related art. The same reference numerals are given to the parts that are the same as those shown in FIG.2 and the descriptions thereof will be omitted.
In the differential amplifying circuit 21 in the related art which ensures input and output linearity, in addition to the circuit of FIG.2, an NPN transistor Q21 is provided between the emitter of the transistor Q1 and the constant-current source 4, and an NPN transistor Q22 is provided between the emitter of the transistor Q2 and the constant-current source 5. The transistor Q21 controls the emitter current of the transistor Q1 in accordance with the emitter voltage of the transistor Q2. The transistor Q22 controls the emitter current of the transistor Q2 in accordance with the emitter voltage of the transistor Q1.
The collector of the transistor Q21 is connected to the emitter of the transistor Q1, the emitter of the transistor Q21 is connected to the connection point of the constant-current source 4 and the resistor R11, and the base of the transistor Q21 is connected to the emitter of the transistor Q2. The collector of the transistor Q22 is connected to the emitter of the transistor Q2, the emitter of the transistor Q22 is connected to the connection point of the constant-current source 5 and the resistor R11, and the base of the transistor Q22 is connected to the connection point of the emitter of the transistor Q1 and the collector of the transistor Q21.
In the above-described differential amplifying circuit 21, the transistors Q21 and Q22 control the base-emitter voltages VBE of the input and output transistors so that the base-emitter voltages do not vary. Thereby, input and output linearity can be ensured.
In the differential amplifying circuit 1 shown in FIG.1 in the related art, the constant-current source 2 and the resistors R1 and R2 are connected to the transistor Q1 in series, and the constant-current source 2 and the resistors R3 and R4 are connected to the transistor Q2 in series. Therefore, it is not possible to reduce the voltage of the differential amplifying circuit. Further, the base-emitter voltages of the transistors Q1 and Q2 vary in accordance with the input voltages. Thereby, input and output linearity cannot be ensured.
In the differential amplifying circuit 11 of low-voltage driving type in the related art shown in FIG.2, the constant-current source 4 is connected to the emitter of the transistor Q1, the constant-current source 5 is connected to the emitter of the transistor Q2 and the resistor R11 for generating a differential voltage is connected between the emitter of the transistor Q1 and the emitter of the transistor Q2. In this arrangement, the difference occurs between the collector current of the transistor Q1 and the collector current of the transistor Q2, and the differential output is output. In the arrangement, the resistors R2 and R4 connected to the transistors Q1 and Q2 in series, respectively, in the circuit shown in FIG.1 can be eliminated. Thereby, it is possible to reduce the voltage of the differential amplifying circuit. However, similar to the differential amplifying circuit 1 shown in FIG., the base-emitter voltages of the transistors Q1 and Q2 vary in accordance with the input voltages. Thereby, input and output linearity cannot be ensured.
In the differential amplifying circuit 21 having linear input and output characteristics in the related art shown in FIG.3, the base-emitter voltages VBE of the input and output transistors are controlled, and thus input and output linearity is ensured. However, because the two transistors are connected to the power source in series, the differential amplifying circuit 21 is not suitable for reducing the voltage of the circuit.